Hi Stephen,
Niello is usually worked on fairly pure silver—so I would be surprised to see a piece that is based on brass with a silver coating. I imagine what you have is chased, and the cuts are filled with dirt and oxidation. I don't see amy telsum in the photo. What do you refer to?
Jamey
The box in the picture attached to the original message... they are unusual as for as I have ever seen telsum but they are indeed little boxes. Stephen.
please see other post!
These are not nielloed, they are chased engraved work, the patina is causing you to mistake it for niello.
Warm wishes
Sarah